1. Field of the Invention
This invention relates to a process for producing a toner for developing electrostatically charged images.
2. Description of the Prior Art
Conventionally, toners for development of electrostatic charges have been manufactured generally by fusion-mixing colorants into thermoplastic resins to be dispersed uniformly therein, followed by pulverization and classification into desired particle sizes by means of a micropulverizer and a classifier. This preparation method is capable of producing considerably excellent toners but limited in certain respects, namely in the scope of choice of the materials for toner. For example, the resin with pigment dispersed therein is required to be sufficiently fragile so as to be micropulverized at an economically feasible production rate. From such a requirement, there is also posed a problem that the pigment-resin dispersion is so fragile that particles with a wide particle size distribution are formed and, especially, a large proportion of minute particles are contained therein, when micropulverization is actually effected at a high rate. However, such a highly fragile material will also frequently suffer from further micropulverization or powder formation during use for development in a copying machine. Other requirements for a toner are to be stable during storage, to be free from agglomeration, to have triboelectric charging characteristic suitable for development, to be capable of forming excellent images, to result in no coverage or contamination of a photosensitive member and further to have a low melting point suitable for thermal fusion. These requirements must be satisfied together with the additional conditions imposed on the toner formation method.
In order to remove the drawbacks of the pulverization method as described above, Japanese Laid-Open Patent Publication No. 17735/1973, British Pat. No. 1,538,787 and others propose to produce toners according to the emulsion or suspension polymerization method. These methods will directly provide toners by dispersing a composition comprising a monomer, a polymerization initiator, a colorant, a magnetic material and others in water. According to these methods, since no pulverization step is included, no fragility for the material is necessary, and a toner is obtained in a spherical shape and is thus excellent in free flowing property. It is also possible to obtain a toner excellent in heat fixing characteristic by controlling polymerization adequately or by use of a crosslinking agent. However, according to emulsion or suspension polymerization, a large amount of a dispersant such as a water-soluble polymeric material or sparlingly soluble inorganic material is required to be used and, even when acid treatment or washing with water is conducted for removal of such a material, there will remain a small quantity of the dispersant on the toner surface. As the result, due to the moisture (humidity) in the air, it will exert an influence on the electroconductivity of the toner thereby to make the triboelectric charging characteristic of the toner markedly unstable In addition, according to these methods, a magnetic material or a colorant is caused to be present ununiformly on the toner particle surface, whereby the triboelectric charging characteristic is also rendered unstable to give no clear copied image.
British Pat. No. 1,538,787 discloses a process for producing colored toner particles through a two-step suspension polymerization. In the second polymerization step of the disclosed process, a water-soluble polymerization initiator is used. When the water-soluble polymerization initiator and/or decomposition products thereof remain in the product toner particles, the toner particles are liable to absorb moisture so that the triboelectric charge of the toner may decrease to below an acceptable level and good images cannot be produced in some cases. Further, as the water-soluble polymerization initiator is not soluble in a monomer such as styrene, acrylates and methacrylates, it is highly possible that the initiator promotes the polymerization of not only an additional monomer adsorbed on the seed particles produced in the first step but also of the additional monomer dissolved or dispersed in water, whereby fine polymer particles having a particle size of below 0.3.mu. can be produced. The thus formed fine polymer particles impart an undesirable effect to the developing characteristic of the toner particles.